fix(core): macos #5122 app.runtime panic in app.set_activation_policy

[washanhanzi]

1. fix [bug] set_actionvation_policy() breaks window.show() #5122
2. the problem:

pub fn run() {
    let mut app = tauri::Builder::default()
        .setup(|app| {
            // c
            app.set_activation_policy(tauri::ActivationPolicy::Accessory);
            Ok(())
        })
        .build(tauri::tauri_build_context!())
        .unwrap();
    // a
    app.set_activation_policy(tauri::ActivationPolicy::Regular);
    // b
    app.run(move |_app_handle, _event| {})
}

a: this line runs first
b: app.run will take the app.runtime as in

tauri/core/tauri/src/app.rs

Line 875 in 00e1567

self.runtime.take().unwrap().run(move |event| match event {

c: the set_activation_policy will try to get app.runtime and unwrap, but now, the app.runtime is None and the program panic.

I add an if let to set_activation_policy. This will cause c ignored silently, maybe we need a better approach.

And it seems the bug is not related to window.show() in the original issue, whether before or after the multi web view refactor c77b403.

[lucasfernog]

Pushed a change to make it work when the app is running too. Thank you!